Recepient failed to export

Sujit Bangera (accounts executive) (23 Points)

23 June 2022  
we have sold the goods to our customer in the month of Feb 22, gst rate 0.1%, since the customer was going to export the goods. but customer has failed to export the goods within 90 days , now customer is ready to pay us 18% gst
plz advise how to account this in our books and how to file the same on gst portal